The overall objectives of the project are to investigate and clarify the pharmacological and physiological interrelationships among calcium and the secretion of calcemic hormones and gastrointestinal (GI) hormones. Prinicipal attention will be focussed on the physiological and pharmacological effects and mechanisms of action of calcium, thyrocalcitonin (TCT), parathyroid hormone (PTH), and gastrin (including related peptides, e.g., cholecystokinin) in an attempt to elucidate the role that these interrelationships might play in health and disease. Problems to be investigated include: (1) Mechanisms by which calcium and GI hormones or factors stimulate TCT secretion as measured by radioimmunoassay either in vivo in pig thyroid vein or rat cardiac blood or in vitro in media used to incubate thyroid glands from 5 to 10-day-old baby rats. (2) Means by which TCT and PTH directly influence secretion of gastrin independently of an effect on blood calcium. Studies to be conducted in vivo in the pig employing antral infusion of test substances and measurement of gastrin in venous blood draining the antrum. (3) Determination of whether calcium, GI hormones, and other known TCT secretagogues affect biosynthesis of TCT by examining repletion of TCT in rat thyroids in vitro after prior depletion of tissue stores of the hormone. (4) Examination of the factors responsible for the mechanisms involved in increased TCT secretion during special altered states of metabolism in the rat. Conditions to be studied include lactation, suckling, chronic ingestion of low iodine diet and increasing age.